Pulp products such as paper and board are manufactured commercially in large scale paper machines. Paper machines generally have a forming section, a press section, a dryer section, a calendering section (or stack) and a take-up reel. The path of the pulp stock through the paper machine from the forming section to the take-up reel is sometimes referred to herein as the paper path.
The forming section comprises endless moving forming fabrics or screens of several well known types (fourdrinier, double wire and cylinder, etc.) onto which a slurry of pulp stock is spread continuously. Water drains from the pulp stock, or is removed under suction, so that a layer of pulp (the "paper sheet") is formed on the forming fabric or screen.
The paper sheet then passes through the press section, where water is removed mechanically by squeezing the paper sheet between large rotating rolls or cylinders, and the dryer section, where the paper sheet is subjected to evaporative drying to further reduce the water content.
On leaving the dryer section, the paper sheet enters the calender section, usually consisting of one or two calender stacks employing hard steel and/or "soft covered" steel rolls in which the steel roll has been coated with plastic or other elastic material to provide a softer surface.
Calendering decreases thickness, increases the density of the paper and improves the paper finish. The paper sheet is then wound into rolls on the take-up reel.
Conventionally, calendering is performed on a vertical stack of rolls or cylinders, generally made of cast iron and having a hardened smooth surface or, in the case of "soft" nip calendering, one hard roll and one soft covered roll. The rolls extend across the width of the paper machine, which can be up to about 10 meters on modern machines. As the paper sheet passes between two adjacent rolls, the weight of the rolls presses on the paper sheet, changing the thickness, density and finish of the paper sheet. The pressure exerted by the calender rolls can, in some calender stacks, be adjusted and additional load added to some rolls to increase the pressure exerted by the rolls, or the weight relieved for some rolls to reduce the pressure exerted by the rolls.
This calendering process continues as the paper sheet proceeds through additional nips between adjacent rolls. On modern machines, the hard nip calender stack generally comprises four to six rolls, resulting in three to five nips between adjacent rolls. Older paper machines generally include a larger number of calender rolls, often nine. Soft calenders generally have two nips, each nip with one hard and one soft covered roll.
The finish imparted by the calendering process is dependent on a number of factors. The most important of these is the calender type (hard or soft) and then the calender load (the pressure exerted by the calender rolls on the paper sheet), however, high calender loads may create weaknesses in the paper sheet. The temperature of the paper sheet is also an important factor and heated calender rolls have been used for many years to improve the paper finish. Finally, other factors include the calender configuration (including the roll diameter), the speed of the paper sheet through the calender, the pulp type and the moisture content of the paper sheet.
Calender rolls were originally heated using steam heating through a small central bore. However, steam heating is relatively inefficient and has been largely replaced by hot water heating which is the most widely used method today.
There are several practical limitations to the use of heated calender rolls. First, a ten degree rise in the calender hot water temperature only results in about a three degree rise in the temperature of the paper sheet. Second, at water temperatures above 100.degree. C., the hot water system must be pressurized to maintain the water in the liquid phase. At typical hot water temperatures of 125.degree. C., the costs of pressurizing the system are acceptable. However, as the temperature rises further, the cost of building and maintaining a pressurized hot water system becomes unacceptably high. Third, at high temperatures, the surface of the calender roll may be distorted through the "oxbow" effect which results in a non-uniform paper thickness across the machine width.
The temperature of the paper sheet reaches a maximum in the dryer section of the paper machine, where heat is applied, and decreases thereafter due to convection and thermal losses to the ambient air.
The heat losses, and consequent temperature drop, in the paper sheet can be dramatic and the temperature drop, for example, between the dryer section and the calender section, or between two calender stacks, can be more than 20.degree. C., high enough that it is difficult to replace the heat through heating of the calender rolls. For example, on a high speed newsprint machine, temperature readings for the paper sheet were recorded as follows: from the dryer section to the first calender stack, the temperature of the paper sheet dropped from 85.degree. C. to 60.degree. C. The calender rolls in the first calender stack were heated and the temperature of the paper sheet on exiting the first calender stack was 68.degree. C. The temperature drop for the paper sheet from the first calender stack to the second calender stack was from 68.degree. C. to 48.degree. C. The calender rolls in the second calender stack were heated and the sheet exited the second calender stack at a temperature of 60.degree. C.
The actual temperature changes in the paper sheet recorded in any given paper machine will depend on many variables such as the paper sheet composition, speed, thickness and the distance the paper sheet travels between the dryer and the calender section or between the calender stacks, and the ambient air temperature.